The use of antiblocking and slip agents for polymer films is well-known in the art. Antiblocking agents lend a fine structure to otherwise smooth film surfaces such that a small air space is maintained between film surfaces, preventing adhesion of two pieces of film to each other. Early antiblocking agents were made of fine powders of inorganic or polymeric materials which were adhered to a film surface or were incorporated into extruded films. Such powders have broad particle size distribution and are typically non-spherical, hence they often impart an undesirable hazy appearance to the film. In recent years, the use of polymeric beads or microspheres has grown. Such microspheres can be produced in narrow particle size distributions and in a variety of shapes (spheres, ovoids, pearls, etc.) to suit particular applications. In addition, by appropriate choice of materials and particle size, such beads are quite transparent on a film surface.
When referring to materials coated onto polymeric sheets, "slip agents," "antiblocking agents," and "antifriction agents" all refer to materials which prevent two sheets from adhering together. These three terms are used interchangeably herein. Typically, polymeric film sheets with smooth surfaces possess a high coefficient of friction such that they tend to "block" or adhere to each other.
Polymeric beads are commercially available, including the Microthene F.TM. brand of polyethylene and ethylene-vinyl acetate beads from U.S.I. Chemicals, Inc., Pergopak M-2.TM. urea-formaldehyde beads from Ciba-Geigy, and Fine Pearl 3000F.TM. polystyrene beads from Matsumoto Co. Polyethylene and urea-formaldehyde beads are not spherical in shape and often provide hazy appearance to films on which they are coated. Polystyrene and poly(methylmethacrylate) beads produce their antiblocking effect mechanically due to the hardness of their surfaces rather than through any lubricating effect.
The production of hard polymethylmethacrylate beads by suspension polymerization methods is well known (U.S. Pat. No. 2,701,245, to duPont). The use of these beads as antiblocking or slip agents in polymer films for transparencies is described in U.S. Pat. No. 4,869,955, to duPont. Microspherical beads made from monofunctional ethylenically-unsaturated monomers are described in U.S. Pat. Nos. 4,614,708 and 4,861,818, both to AGFA-Gevaert. These beads are used in an antifriction layer of a photographic element. No quantification of the antifriction properties of these beads is presented.
Long-chain (C.sub.12 to C.sub.30) hydrocarbons have long been recognized as lubricants, especially long-chain aliphatic carboxylic acids and esters. For example, stearic acid derivatives and stearyl esters are well-known and enjoy wide use as additives to coatings or to polymer constructions where lubricity is desired. The use of other fatty acids and fatty acid esters, including polymers comprising such acids and esters, as lubricants is well known in the art. However, polymeric beads comprising long-chain aliphatic moeities are not well known. Japanese Patent Application 87 231957 (Konica Co.) discloses an uncrosslinked latex of stearyl acrylate used in photographic film constructions. Good antiblocking properties are indicated, although the sub-micron sized latex produced by the emulsion polymerization process described would form a film when coated rather than discreet particles.
Thus, commercially-available microspheres comprise polymers of monofunctional monomers, such as methyl methacrylate, styrene, ethylene, and tetrafluoroethylene. Difunctional acrylates such as butanediol di(meth)acrylate, hexanediol di(meth)acrylate, and related short-chain diol di(meth)acrylates have been used to encapsulate pigment particles by suspension polymerization methods as described in European Patent Application 379 122, to Dainichi Seika. These beads are pigmented and used as polymeric coloring agents. U.S. Pat. No. 4,277,536, to Bayer AG, discloses polymer beads from two highly viscous (500 to 500,000 mPa.s) diacrylate monomers: the bis-glycidyl methacrylate of Bisphenol A, known as bis-GMA; and the bis-hydroxylaklydiacrylate of 1,9-diisocyanatononane. The beads are used as dental fillers. U.S. Pat. No. 4,414,278 describes production of highly solvent-stable spherical microbeads from crosslinked triacrylate or tetraacrylate monomers and copolymers of the tri- or tetra-acrylates with up to 50 percent by weight of comonomers having two terminal ethylenic groups. No polymeric beads of purely diacrylate monomers are disclosed or claimed, and no antifriction properties of the beads are described.
A need exists for microsphere slip or antiblocking agents which can be coated on polymeric sheets such as those used for overhead transparencies which impart a significantly lower coefficient of friction compared to known slip or antiblocking agents.